Update documentation for "max_shingle_size" parameter

[sebelga]

While building the Kibana UI for the mappings editor search_as_you_type type, CJ and Gail improved the description text (see comment) to explain what the max_shingle_size parameter is for.

We should port that effort to the Elasticsearch docs and help users that only go to the docs and don't use the UI.

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)